Hydrostatic transmissions have many uses, including the propelling of vehicles, such as mowing machines. A typical hydrostatic transmission system includes a variable displacement main hydraulic pump connected in a closed hydraulic circuit with a fixed displacement hydraulic motor. For most applications, the pump is driven by a prime mover, such as an internal combustion engine or an electrical motor, at a certain speed in a certain direction. Changing the displacement of the main pump will change its output flow rate, which controls the speed of the motor. Pump outflow can be reversed, thus reversing the direction of the motor.
In some vehicles, such as zero-turn-radius mowers, separate hydraulic pumps and motors are used to independently drive separate wheels of an axle. By independently driving the wheels in opposite directions, for example, the vehicle can be made to turn with zero radius. Zero-turn-radius mowers are increasingly popular as the size and costs of such mowers decrease. As the size of such mowers decreases, however, the space available for the hydraulic components and/or the prime mover also decreases.
Hydrostatic transmissions generate heat as the hydraulic fluid is circulated between the pump and the motor. Friction between moving parts of the pump and/or motor also generates heat. Consequently, heat exchangers have been employed to cool the hydraulic fluid. In addition, filters have been used to filter the hydraulic fluid to remove contaminants that become entrained in the fluid during use.
In present day equipment, the heat exchanger, filter assembly and hydraulic fluid reservoir are provided as individual system components that are connected together by hydraulic hoses and fittings. These connections are potential leak paths, and the multiple components must be separately stocked.